Field of the Invention
The invention relates to an electromagnetic steel sheet having an electrically insulating film which excels in corrosion resistance and, more particularly, to an electromagnetic steel sheet having an electrically insulating film of phosphate-organic resin type.
Insulating films on electromagnetic steel sheets are required to meet various requirements such as electrical insulation, closeness of adhesion, punchability, weldability, corrosion resistance, and so forth. Studies have been made to satisfy these requirements, and many methods for forming insulating film on the surface of the electromagnetic steel sheet, as well as insulating film compositions, have been proposed.
Japanese Patent Laid-Open No. 50-103438 discloses a method for forming such an insulating film composition of the type which is prepared by compounding an organic resin into a treating solution, wherein the insulating film is formed by using a treating liquid which is prepared by compounding 0.1 to 50 weight parts of an acid-soluble organic resin such as polyvinylalcohol, polyacrylic acid or acrylester emulsion to 10 weight parts of phosphoric acid ion.
Meanwhile, Japanese Patent Laid-Open No. 60-169567 discloses a method in which an insulating film is formed by compounding an acrylic resin, ethylene-vinylacetate copolymer or a thermosetting epoxy resin emulsion to a treating liquid containing phosphate.
Both of these methods utilize treating liquids which are prepared by compounding organic resins to a solution mainly composed of phosphate. These methods require baking at temperatures exceeding 300.degree. C. Consequently, the resin thermally decomposes during the firing, resulting in inferior coating film performance in terms of corrosion resistance and punchability, as well as insufficient shelving stability of the above-mentioned emulsion in the phosphate type aqueous solution.
More specifically, the resins used in Japanese Patent Laid-Open Nos. 50-103438 and 60-169567 are selected primarily for their dissolution or dispersion properties in the treating liquid, while the heat resistance of the resin is ignored. Consequently, the electromagnetic sheets exhibit inferior corrosion resistance.
We have proposed, in Japanese Patent Laid-Open No. 6-235070, an electromagnetic steel sheet which excels both in weldability and punchability, as well as in corrosion resistance. This electromagnetic steel sheet is produced by using a resin primarily composed of a chromate and having a thermal decomposition peak temperature of 400.degree. C. or higher. More specifically, in this method, emulsion resin particles have a two-layered structure composed of an outer layer constituted by a resin having high resistance to chromic acid and a thermal decomposition peak temperature of about 400.degree. C. or higher, and an inner layer constituted by a resin which is crosslinkable so as to enhance heat resistance and which has a thermal decomposition peak temperature of about 400.degree. C. or higher, thus improving stability and heat resistance of the coating solution.
In recent years, however, a trend toward automated production of cores such as motor cores has given rise to a demand for greater slipperiness of the steel before and after strain relieving annealing. The term "slipperiness" is used herein to describe a characteristic of the steel that enables the steel to smoothly slip onto a guide or other steel sheet. This characteristic enables smooth transportation of steel sheets, as well as smooth lamination of steel sheets punched in the form of motor cores.
A steel coated with a resin containing chromate as the principal component exhibits inferior slipperiness. Further, there is a current demand for chromium-free materials in order to prevent environmental pollution. The method disclosed in Japanese Patent Laid-Open No. 6-235070 inevitably increases running costs due to the expense associated with disposal of waste liquids, the preservation of a good working environment, and so forth. The use of chromate as the primary component of the insulating film composition is therefore not recommended.